1. Field of the Invention
The present invention relates generally to analyzing the modal behavior of materials for the purpose of detecting the presence or absence of residual stresses, and more particularly to a method and apparatus for relieving residual stresses in a workpiece by vibrating the workpiece.
2. Description of Related Art
Metal working procedures such as forging, casting, welding, machining and the like introduce residual stresses into workpieces. Release of residual stresses, often described as creep or lattice slip, occurs naturally over time as a workpiece ages, in a process known as stress-directed atomic diffusion, whereby natural tensile and compressive stresses equalize and residual stresses are released. If sufficient time is not allowed for such diffusion to occur in a workpiece, these residual stresses can result in premature failure of the workpiece, an increased tendency of the workpiece to warp or be subject to heat-checking, as well as an increase in the rate of corrosion of the workpiece. Accordingly, various procedures have been used to relieve residual stresses. Such procedures include hot working, step aging, precipitation aging, over-aging, quench aging, progressive aging, heat treatment, cryogenics, cold working operations such as strain aging, shot-peening, and alternating magnetic field excitation. While these techniques have been known in the art, all are fraught with one or more drawbacks including, but not limited to, the cost of equipment and operation, the size of equipment, and their effectiveness in relieving residual stresses. In addition, these techniques generally do not include as an inherent part of their method the ability to verify effective release of residual stresses. Also, in all these methods, comparative measurement of the actual properties of workpieces prior to and after treatment has hitherto been generally limited to costly destructive testing, surface-limited and non-conclusive x-ray diffraction or magnetoelastic surface analysis techniques, or empirical observation of dimensional or other changes (or the lack thereof) during machining or in use.
It is also known that the application of a cyclic vibrational force to a workpiece also relieves residual stresses. Such a method for relieving stress in a workpiece by vibration is described in U.S. Pat. No. 4,968,359. The method described in this reference involves relieving stress by initially applying a vibrational force over a test frequency range, monitoring the dampening effects, identifying a plurality of orders of harmonic vibration absorption peaks, and then applying a vibrating force at a fixed frequency corresponding to a sub-harmonic frequency of one of the harmonic peaks. Another method for relieving stress is described in U.S. Pat. No. 3,741,820 which also involves the vibration of the workpiece at a fixed frequency. Another method for relieving stresses by applying vibrational force is described in U.S. Pat. No. 4,381,673 which involves vibrating at fixed resonant frequencies.
While prior art techniques for relieving stress by applying vibrational forces have generally proven effective, they also have several disadvantages. For example, prior techniques generally do not vary the frequency of the vibration in response to changes in the frequency response of the workpiece as residual stresses are relieved. In addition, such methods heretofore have relied exclusively on measurement of the frequency response of the workpiece by means of a ramp-up of vibrational energy introduced into the workpiece with a series-wound motor-driven exciter. If precise motor speed control is not maintained, these methods and apparati may be inaccurate, causing someone not well-schooled in the art to be misled by false frequency locations caused by operating temperature variations in the exciter motor.